Using IRIS transition region lines (O IV, S IV and Si IV) electron density may be computed ([1]}; [2]}; [3]}). We note that in the spectra corresponding to the reconnection site at the reconnection time, O IV lines are
detected, even the emission is relatively weak (Figs. REF , REF  top panels and REF ). Si IV line profiles vary drastically according to time or location as we have already mentioned in Sect. REF .
The great variety of shapes of profiles of the Si IV lines
rises a question about the variations of the optical thickness throughout the observed mini-flare area. For this investigation, we employ the method involving
the intensity ratio of the Si IV 1393.75 Å and 1402.77 Å resonance lines ([4]}; [5]}).
Since a long time this
technique exists for stars to determine the amount of opacity in the Si IV lines and is very powerful
for providing the physical dimensions of the scattering layer
[6]}. For computing the intensity Si IV ratio we select two observing times
: one time during reconnection (02:04:28 UT) with slit 1 at the reconnection location (Fig. REF ), and the other time at one minute later (02:05:36 UT) in slit 4 at the jet base (6\(\mbox{$~\!\!^{\prime \prime }$}\)  away from the reconnection point).
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